fix mg_send() failing on 0 length

mongoose.c

@@ -6225,9 +6225,9 @@ bool mg_send(struct mg_connection *c, const void *buf, size_t len) {
     len = trim_len(c, len);  // Trimming length if necessary
     res = udp_send(c, buf, len);
   } else {
-    res = (bool) mg_iobuf_add(&c->send, c->send.len, buf, len);
+    res = len == 0 || mg_iobuf_add(&c->send, c->send.len, buf, len) > 0;
-    // res == 0 means an OOM condition (iobuf couldn't resize), yet this is so
+    // returning 0 means an OOM condition (iobuf couldn't resize), yet this is
-    // far recoverable, let the caller decide
+    // so far recoverable, let the caller decide
   }
   return res;
 }
@@ -9582,7 +9582,7 @@ bool mg_send(struct mg_connection *c, const void *buf, size_t len) {
     iolog(c, (char *) buf, n, false);
     return n > 0;
   } else {
-    return (bool) mg_iobuf_add(&c->send, c->send.len, buf, len);
+    return len == 0 || mg_iobuf_add(&c->send, c->send.len, buf, len) > 0;
     // returning 0 means an OOM condition (iobuf couldn't resize), yet this is
     // so far recoverable, let the caller decide
   }

src/net_builtin.c

@@ -2019,9 +2019,9 @@ bool mg_send(struct mg_connection *c, const void *buf, size_t len) {
     len = trim_len(c, len);  // Trimming length if necessary
     res = udp_send(c, buf, len);
   } else {
-    res = (bool) mg_iobuf_add(&c->send, c->send.len, buf, len);
+    res = len == 0 || mg_iobuf_add(&c->send, c->send.len, buf, len) > 0;
-    // res == 0 means an OOM condition (iobuf couldn't resize), yet this is so
+    // returning 0 means an OOM condition (iobuf couldn't resize), yet this is
-    // far recoverable, let the caller decide
+    // so far recoverable, let the caller decide
   }
   return res;
 }

src/sock.c

@@ -141,7 +141,7 @@ bool mg_send(struct mg_connection *c, const void *buf, size_t len) {
     iolog(c, (char *) buf, n, false);
     return n > 0;
   } else {
-    return (bool) mg_iobuf_add(&c->send, c->send.len, buf, len);
+    return len == 0 || mg_iobuf_add(&c->send, c->send.len, buf, len) > 0;
     // returning 0 means an OOM condition (iobuf couldn't resize), yet this is
     // so far recoverable, let the caller decide
   }

test/mip_tap_test.c

@@ -413,6 +413,7 @@ static void test_http_server(struct mg_mgr *mgr) {
       mg_mprintf("./mip_curl.sh http://%M:12347", mg_print_ip4, &mgr->ifp->ip);
 #endif
   ASSERT(c != NULL);
+  ASSERT (mg_send(c, "NADA", 0)); // check mg_send allows len=0
   pthread_create(&thread_id, NULL, poll_thread,
                  mgr);  // simpler this way, no concurrency anyway
   MG_DEBUG(("CURL"));

test/unit_test.c

@@ -2977,6 +2977,14 @@ static void eZ(struct mg_connection *c, int ev, void *ev_data) {
   (void) ev_data;
 }
 
+static void eS(struct mg_connection *c, int ev, void *ev_data) {
+  if (ev == MG_EV_HTTP_MSG) {
+    ASSERT (mg_send(c, "NADA", 0));
+    mg_http_reply(c, 200, "", "abcd");
+  }
+  (void) ev_data;
+}
+
 // Do not delete chunks as they arrive
 static void eh4(struct mg_connection *c, int ev, void *ev_data) {
   uint32_t *crc = (uint32_t *) c->fn_data;
@@ -3012,6 +3020,9 @@ static void test_http_chunked_case(mg_event_handler_t s, mg_event_handler_t c,
 }
 
 static void test_http_chunked(void) {
+  // test mg_send allows calls with 0 length
+  test_http_chunked_case(eS, eh4, 1, "abcd");
+
   // Non-chunked encoding
   test_http_chunked_case(eY, eh4, 1, "abcd");  // Chunks not deleted
   test_http_chunked_case(eY, eh4, 2, "abcdabcd");